Re: [Boost-users] [fusion] sequence conversion

From: Zeljko Vrba (zvrba_at_[hidden])
Date: 2008-07-21 11:02:02

On Mon, Jul 21, 2008 at 03:27:39PM +0200, Istvan Buki wrote:
> 
> InMetaData contain, among other things, the type of the input data. More
> specifically, InMetaData::type is a fusion map describing all the fields of
> the input data.
>
Given that you're using a map, I guess that the user has to define "dummy"
types as field names (unless he's satisfied with exactly one value for each
primitive type)?

Hm, a crazy question: what would happen if I decided to declare

map<pair<int, char>, pair<char, char>, pair<int, double> >

(note the repeated use of int key)..  (That was a rhetoric question, I
can try that myself :-))

> 
>        typename result_of::join< InMetaData, CacheMetaData, IndexType
> >::type
>       apply( const typename InMetaData::type &r )
>       {
>     typename IndexType::value_type key_value =
>       fusion::at_key< IndexType >( r ) ;
>     return fusion::as_map( fusion::join( r, fusion::erase_key< IndexType >(
> cache_.lookup( key_value ) ) ) ) ;
>       }
> 
Given your explanation, I have several questions about this code:

  - the name apply -- is this function a part of some larger class, so the name
  is prescribed by some protocol?

  - You have CacheMetaData as template parameter, yet you do not use it -- you
  have an instance variable cache_ instead.  Is this just for illustration
  purposes or some other reason?

  - the return statement prejudices "map" as implementation, i.e., the code
  does not look generic enough :-)  is there some metafunction that could
  replace fusion::as_map with something more generic, in case the user would
  like to use e.g. vectors?  ditto for erase_key?

And now a perceived inconsistency within fusion itself -- vector is clearly a
map from a set of integers [0..n-1] to types, yet it is not a model of
associative sequence.  I.e. after having read the documentation for erase_key,
it turns out that for vectors one has to use erase(), but erase_key() for
maps..  i'm not saying that it's good or bad, it just strikes me as weird.

Could you please also write a simple example of composition of two elements?
(I.e. how it would be used by the 'end-user' programmer?) 

> 
> On the other hand if I work by ignoring what is not needed I need explicitly
> to tell what I'll copy to the output file which requires more work. In fact
>
Yes, the same reason occurred to me a bit later too.

>
> something concrete which gives me a clear goal. I'll try to use fusion and
> MPL as much as possible and see how far I can go with them. At this point I
>
nice attitude :)

> 
> I hope this can be of some help.
>
Yes, it was helpful.  Thank you.